Structural nonuniformities in vehicle tires can give rise to an undesirable phenomenon known as "steering pull." Steering pull refers to the tendency of a vehicle traveling forward to veer to the right or left in the absence of a corresponding steering input from its driver.
The steering pull exhibited by a vehicle due to non-uniformity of its tires is attributable almost entirely to behavior of the tires mounted on the steering axle. The tendency of a tire on one end of the axle to cause steering pull algebraically adds to or subtracts from the corresponding tendency of the tire on the other end of the axle. In order to illustrate this, it is useful to first note that tires have a pair of opposed sides which are commonly referred to as the "in-side" (or D.O.T. marking side) and the "out-side" (sometimes referred to as "curb side" or "whitewall side"). Assume for illustration purposes that the tire mounted on the left side of the front axle of a vehicle, such as passenger car, tends to veer toward its out-side to such a degree that in order to keep the vehicle traveling straight, a 5 ounce clockwise steering pull force must be applied by the driver to the perimeter of the steering wheel of the car. Further assume that the tire on the opposite end of the same axle tends to pull in the opposite direction, i.e., toward the in-side, with a steering pull of the same magnitude. Since the tires are mounted at opposite ends of the axle, each would tend to cause the car to tend to veer leftwardly and, in order to keep the vehicle traveling straight, the driver would have to exert a distinctly noticeable total steering pull of 10 ounces on the steering wheel in the clockwise direction. However, if only one of these tires was replaced with a tire exhibiting the same degree of steering pull but in the opposite direction of its predecessor, i.e., in the same direction as the tire on the opposite end of the steering axle, the driver would not have to exert any force on the steering wheel to keep the vehicle traveling straight ahead. No annoying steering pull would then be apparent to the driver notwithstanding the tendency of each individual tire to cause steering pull.
Steering pull can be quantified by measuring the magnitude and direction of force which the driver must apply at the perimeter of the steering wheel in one direction or the other in order to keep the vehicle traveling straight ahead.
To reduce customer complaints and resolve warranty claims, manufacturers of tires and vehicles need to be able to determine whether non-uniformities in the tires mounted on a vehicle give rise to an unacceptable amount of steering pull. However, until the advent of the present invention, accurate resolution of the steering pull exhibited by a vehicle due to tire non-uniformities has been problematic because factors other than tire non-uniformities are reflected in the total pull apparent at the steering wheel of the vehicle. For example, the arched cross-section or "crown" formed in many roads to facilitate water drainage influences the steering pull detectable at the steering wheel of the vehicle. These influences can be negated by making measurements while the vehicle runs on a flat surface. In situations where a suitable flat surface is unavailable, it has been known to correct for measurement errors induced by road crowns by driving the vehicle in opposite directions over the same crowned road surface and then averaging the apparent algebraic steering pull measured over those equal and opposite legs of travel.
A more vexing problem has been the error introduced in steering pull measurements due to the influence of crosswinds acting on the vehicle. A crosswind in a given direction tends to algebraicly increase the steering pull in that direction as measured at the steering wheel of the vehicle. The magnitude of the increase depends on the magnitude of the crosswind, its direction relative the direction of travel of the vehicle and other factors such as the projected cross-sectional area and aerodynamic coefficient of the particular vehicle in the direction of the crosswind. Steering pull measurement error due to crosswinds can be avoided or reduced by making measurements only when winds are suitably calm, but doing so is not always possible or convenient.
It has been attempted to reduce steering pull measurement error induced by crosswinds by averaging the algebraic steering pull values measured at the steering wheel when the vehicle is driven in two opposing directions so that the net influence of crosswinds tends to average toward zero. This technique is not reliable however as the speed and/or direction of crosswinds cannot be relied upon to remain constant while measurements are taken in opposing directions of vehicle travel. Even if prevailing winds remain constant, their apparent direction with respect to the vehicle is subject to change with changes in the direction of the vehicle due to changes in the direction of the course over which it is driven as steering pull measurements are made.